Photoacid generators (PAG) used in photoresists are being designed to generate bulkier anions in order to suppress acid diffusion during post exposure bake (PEB), and hence provide higher resolution. However, this trend often causes higher defectivity as bulkier anions are hydrophobic, often containing lipophilic groups such as fluorinated groups, and may have poor solubility in developer and in rinse water.
One way to simultaneously achieve low diffusivity and good defectivity is to increase the size of PAG anion through attaching acid-cleavable groups. By doing so, the size of PAG anion can be large enough to suppress acid diffusion at the time of PEB and fragmented into smaller, highly polar species upon completion of the PEB process. The cleaved photoacid dissolves readily upon contacting with tetramethylammonium hydroxide (TMAH) developers which can reduce the number of defects. Acid cleavability can be obtained from the use of acetal or ketal linkages on the photogenerated anion moiety.
Ketal-protected polyhydroxy anions have been described in Japanese Patent No. JP2011201860(A), which are based on fluorinated adducts of a specific polyhydroxy tetrahydrofuranyl ring system. However, there remains a need for photoacid generators for 193 nm lithography, which exhibit improved resolution and defect control.